Transistors formed over a glass substrate or the like have been manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors using amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. Transistors using polycrystalline silicon have high field-effect mobility, but they are not suitable for a larger glass substrate.
Instead of a transistor using silicon, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and such a transistor is used as a switching element or the like of a pixel of a display device.